Async-Interrupt/Interrupt.pm

=head1 NAME

Async::Interrupt - allow C/XS libraries to interrupt perl asynchronously

=head1 SYNOPSIS

 use Async::Interrupt;

=head1 DESCRIPTION

This module implements a single feature only of interest to advanced perl
modules, namely asynchronous interruptions (think "UNIX signals", which
are very similar).

Sometimes, modules wish to run code asynchronously (in another thread,
or from a signal handler), and then signal the perl interpreter on
certain events. One common way is to write some data to a pipe and use an
event handling toolkit to watch for I/O events. Another way is to send
a signal. Those methods are slow, and in the case of a pipe, also not
asynchronous - it won't interrupt a running perl interpreter.

This module implements asynchronous notifications that enable you to
signal running perl code from another thread, asynchronously, and
sometimes even without using a single syscall.

=head2 USAGE SCENARIOS

=over 4

=item Race-free signal handling

There seems to be no way to do race-free signal handling in perl: to
catch a signal, you have to execute Perl code, and between entering the
interpreter C<select> function (or other blocking functions) and executing
the select syscall is a small but relevant timespan during which signals
will be queued, but perl signal handlers will not be executed and the
blocking syscall will not be interrupted.

You can use this module to bind a signal to a callback while at the same
time activating an event pipe that you can C<select> on, fixing the race
completely.

This can be used to implement the signal hadling in event loops,
e.g. L<AnyEvent>, L<POE>, L<IO::Async::Loop> and so on.

=item Background threads want speedy reporting

Assume you want very exact timing, and you can spare an extra cpu core
for that. Then you can run an extra thread that signals your perl
interpreter. This means you can get a very exact timing source while your
perl code is number crunching, without even using a syscall to communicate
between your threads.

For example the deliantra game server uses a variant of this technique
to interrupt background processes regularly to send map updates to game
clients.

L<IO::AIO> and L<BDB> could also use this to speed up result reporting.

=item Speedy event loop invocation

One could use this module e.g. in L<Coro> to interrupt a running coro-thread
and cause it to enter the event loop.

Or one could bind to C<SIGIO> and tell some important sockets to send this
signal, causing the event loop to be entered to reduce network latency.

=back

=head2 HOW TO USE

You can use this module by creating an C<Async::Interrupt> object for each
such event source. This object stores a perl and/or a C-level callback
that is invoked when the C<Async::Interrupt> object gets signalled. It is
executed at the next time the perl interpreter is running (i.e. it will
interrupt a computation, but not an XS function or a syscall).

You can signal the C<Async::Interrupt> object either by calling it's C<<
->signal >> method, or, more commonly, by calling a C function. There is
also the built-in (POSIX) signal source.

The C<< ->signal_func >> returns the address of the C function that is to
be called (plus an argument to be used during the call). The signalling
function also takes an integer argument in the range SIG_ATOMIC_MIN to
SIG_ATOMIC_MAX (guaranteed to allow at least 0..127).

Since this kind of interruption is fast, but can only interrupt a
I<running> interpreter, there is optional support for signalling a pipe
- that means you can also wait for the pipe to become readable (e.g. via
L<EV> or L<AnyEvent>). This, of course, incurs the overhead of a C<read>
and C<write> syscall.

=over 4

=cut

package Async::Interrupt;

no warnings;

BEGIN {
   $VERSION = '0.041';

   require XSLoader;
   XSLoader::load Async::Interrupt::, $VERSION;
}

our $DIED = sub { warn "$@" };

=item $async = new Async::Interrupt key => value...

Creates a new Async::Interrupt object. You may only use async
notifications on this object while it exists, so you need to keep a
reference to it at all times while it is used.

Optional constructor arguments include (normally you would specify at
least one of C<cb> or C<c_cb>).

=over 4

=item cb => $coderef->($value)

Registers a perl callback to be invoked whenever the async interrupt is
signalled.

Note that, since this callback can be invoked at basically any time, it
must not modify any well-known global variables such as C<$/> without
restoring them again before returning.

The exceptions are C<$!> and C<$@>, which are saved and restored by
Async::Interrupt.

If the callback should throw an exception, then it will be caught,
and C<$Async::Interrupt::DIED> will be called with C<$@> containing
the exception.  The default will simply C<warn> about the message and
continue.

=item c_cb => [$c_func, $c_arg]

Registers a C callback the be invoked whenever the async interrupt is
signalled.

The C callback must have the following prototype:

   void c_func (pTHX_ void *c_arg, int value);

Both C<$c_func> and C<$c_arg> must be specified as integers/IVs, and
C<$value> is the C<value> passed to some earlier call to either C<$signal>
or the C<signal_func> function.

Note that, because the callback can be invoked at almost any time, you
have to be careful at saving and restoring global variables that Perl
might use (the exception is C<errno>, which is saved and restored by
Async::Interrupt). The callback itself runs as part of the perl context,
so you can call any perl functions and modify any perl data structures (in
which case the requirements set out for C<cb> apply as well).

=item signal => $signame_or_value

When this parameter is specified, then the Async::Interrupt will hook the
given signal, that is, it will effectively call C<< ->signal (0) >> each time
the given signal is caught by the process.

Only one async can hook a given signal, and the signal will be restored to
defaults when the Async::Interrupt object gets destroyed.

=item pipe => [$fileno_or_fh_for_reading, $fileno_or_fh_for_writing]

Specifies two file descriptors (or file handles) that should be signalled
whenever the async interrupt is signalled. This means a single octet will
be written to it, and before the callback is being invoked, it will be
read again. Due to races, it is unlikely but possible that multiple octets
are written. It is required that the file handles are both in nonblocking
mode.

You can get a portable pipe and set non-blocking mode portably by using
e.g. L<AnyEvent::Util> from the L<AnyEvent> distribution.

It is also possible to pass in a linux eventfd as both read and write
handle (which is faster than a pipe).

The object will keep a reference to the file handles.

This can be used to ensure that async notifications will interrupt event
frameworks as well.

=back

=cut

sub new {
   my ($class, %arg) = @_;

   bless \(_alloc $arg{cb}, @{$arg{c_cb}}[0,1], @{$arg{pipe}}[0,1], $arg{signal}), $class
}

=item ($signal_func, $signal_arg) = $async->signal_func

Returns the address of a function to call asynchronously. The function has
the following prototype and needs to be passed the specified C<$c_arg>,
which is a C<void *> cast to C<IV>:

   void (*signal_func) (void *signal_arg, int value)

An example call would look like:

   signal_func (signal_arg, 0);

The function is safe to call from within signal and thread contexts, at
any time. The specified C<value> is passed to both C and Perl callback.

C<$value> must be in the valid range for a C<sig_atomic_t> (0..127 is
portable).

If the function is called while the Async::Interrupt object is already
signaled but before the callbacks are being executed, then the stored
C<value> is either the old or the new one. Due to the asynchronous
nature of the code, the C<value> can even be passed to two consecutive
invocations of the callback.

=item $async->signal ($value=0)

This signals the given async object from Perl code. Semi-obviously, this
will instantly trigger the callback invocation.

C<$value> must be in the valid range for a C<sig_atomic_t> (0..127 is
portable).

=item $async->block

=item $async->unblock

Sometimes you need a "critical section" of code that will not be
interrupted by an Async::Interrupt. This can be implemented by calling C<<
$async->block >> before the critical section, and C<< $async->unblock >>
afterwards.

Note that there must be exactly one call of C<unblock> for every previous
call to C<block> (i.e. calls can nest).

Since ensuring this in the presence of exceptions and threads is
usually more difficult than you imagine, I recommend using C<<
$async->scoped_block >> instead.

=item $async->scope_block

This call C<< $async->block >> and installs a handler that is called when
the current scope is exited (via an exception, by canceling the Coro
thread, by calling last/goto etc.).

This is the recommended (and fastest) way to implement critical sections.

=item $async->pipe_enable

=item $async->pipe_disable

Enable/disable signalling the pipe when the interrupt occurs (default is
enabled). Writing to a pipe is relatively expensive, so it can be disabled
when you know you are not waiting for it (for example, with L<EV> you
could disable the pipe in a check watcher, and enable it in a prepare
watcher).

Note that when C<fd_disable> is in effect, no attempt to read from the
pipe will be done.

=cut

1;

=back

=head1 EXAMPLE

There really should be a complete C/XS example. Bug me about it. Better
yet, create one.

=head1 IMPLEMENTATION DETAILS AND LIMITATIONS

This module works by "hijacking" SIGKILL, which is guaranteed to always
exist, but also cannot be caught, so is always available.

Basically, this module fakes the occurance of a SIGKILL signal and
then intercepts the interpreter handling it. This makes normal signal
handling slower (probably unmeasurably, though), but has the advantage
of not requiring a special runops function, nor slowing down normal perl
execution a bit.

It assumes that C<sig_atomic_t> and C<int> are both async-safe to modify
(C<sig_atomic_> is used by this module, and perl itself uses C<int>, so we
can assume that this is quite portable, at least w.r.t. signals).

=head1 AUTHOR

 Marc Lehmann <schmorp@schmorp.de>
 http://home.schmorp.de/

=cut

Revision:	1.9
Committed:	Sun Jul 12 16:33:26 2009 UTC (14 years, 11 months ago) by root
Branch:	MAIN
CVS Tags:	rel-0_041
Changes since 1.8:	+1 -1 lines
Log Message:	0.041
#	User	Rev	Content
1	root	1.1	=head1 NAME
2
3			Async::Interrupt - allow C/XS libraries to interrupt perl asynchronously
4
5			=head1 SYNOPSIS
6
7			use Async::Interrupt;
8
9			=head1 DESCRIPTION
10
11			This module implements a single feature only of interest to advanced perl
12	root	1.4	modules, namely asynchronous interruptions (think "UNIX signals", which
13	root	1.1	are very similar).
14
15	root	1.8	Sometimes, modules wish to run code asynchronously (in another thread,
16			or from a signal handler), and then signal the perl interpreter on
17			certain events. One common way is to write some data to a pipe and use an
18			event handling toolkit to watch for I/O events. Another way is to send
19			a signal. Those methods are slow, and in the case of a pipe, also not
20			asynchronous - it won't interrupt a running perl interpreter.
21	root	1.1
22			This module implements asynchronous notifications that enable you to
23	root	1.8	signal running perl code from another thread, asynchronously, and
24			sometimes even without using a single syscall.
25	root	1.1
26	root	1.8	=head2 USAGE SCENARIOS
27
28			=over 4
29
30			=item Race-free signal handling
31
32			There seems to be no way to do race-free signal handling in perl: to
33			catch a signal, you have to execute Perl code, and between entering the
34			interpreter C<select> function (or other blocking functions) and executing
35			the select syscall is a small but relevant timespan during which signals
36			will be queued, but perl signal handlers will not be executed and the
37			blocking syscall will not be interrupted.
38
39			You can use this module to bind a signal to a callback while at the same
40			time activating an event pipe that you can C<select> on, fixing the race
41			completely.
42
43			This can be used to implement the signal hadling in event loops,
44			e.g. L<AnyEvent>, L<POE>, L<IO::Async::Loop> and so on.
45
46			=item Background threads want speedy reporting
47
48			Assume you want very exact timing, and you can spare an extra cpu core
49			for that. Then you can run an extra thread that signals your perl
50			interpreter. This means you can get a very exact timing source while your
51			perl code is number crunching, without even using a syscall to communicate
52			between your threads.
53
54			For example the deliantra game server uses a variant of this technique
55			to interrupt background processes regularly to send map updates to game
56			clients.
57
58			L<IO::AIO> and L<BDB> could also use this to speed up result reporting.
59
60			=item Speedy event loop invocation
61
62			One could use this module e.g. in L<Coro> to interrupt a running coro-thread
63			and cause it to enter the event loop.
64
65			Or one could bind to C<SIGIO> and tell some important sockets to send this
66			signal, causing the event loop to be entered to reduce network latency.
67
68			=back
69
70			=head2 HOW TO USE
71
72			You can use this module by creating an C<Async::Interrupt> object for each
73			such event source. This object stores a perl and/or a C-level callback
74			that is invoked when the C<Async::Interrupt> object gets signalled. It is
75			executed at the next time the perl interpreter is running (i.e. it will
76			interrupt a computation, but not an XS function or a syscall).
77	root	1.2
78			You can signal the C<Async::Interrupt> object either by calling it's C<<
79	root	1.8	->signal >> method, or, more commonly, by calling a C function. There is
80			also the built-in (POSIX) signal source.
81	root	1.2
82			The C<< ->signal_func >> returns the address of the C function that is to
83			be called (plus an argument to be used during the call). The signalling
84			function also takes an integer argument in the range SIG_ATOMIC_MIN to
85			SIG_ATOMIC_MAX (guaranteed to allow at least 0..127).
86
87			Since this kind of interruption is fast, but can only interrupt a
88	root	1.8	I<running> interpreter, there is optional support for signalling a pipe
89			- that means you can also wait for the pipe to become readable (e.g. via
90			L<EV> or L<AnyEvent>). This, of course, incurs the overhead of a C<read>
91			and C<write> syscall.
92	root	1.2
93	root	1.1	=over 4
94
95			=cut
96
97			package Async::Interrupt;
98
99	root	1.2	no warnings;
100
101	root	1.1	BEGIN {
102	root	1.9	$VERSION = '0.041';
103	root	1.1
104			require XSLoader;
105			XSLoader::load Async::Interrupt::, $VERSION;
106			}
107
108	root	1.2	our $DIED = sub { warn "$@" };
109
110	root	1.1	=item $async = new Async::Interrupt key => value...
111
112			Creates a new Async::Interrupt object. You may only use async
113			notifications on this object while it exists, so you need to keep a
114			reference to it at all times while it is used.
115
116			Optional constructor arguments include (normally you would specify at
117			least one of C<cb> or C<c_cb>).
118
119			=over 4
120
121			=item cb => $coderef->($value)
122
123			Registers a perl callback to be invoked whenever the async interrupt is
124			signalled.
125
126			Note that, since this callback can be invoked at basically any time, it
127	root	1.2	must not modify any well-known global variables such as C<$/> without
128			restoring them again before returning.
129
130			The exceptions are C<$!> and C<$@>, which are saved and restored by
131			Async::Interrupt.
132	root	1.1
133	root	1.2	If the callback should throw an exception, then it will be caught,
134			and C<$Async::Interrupt::DIED> will be called with C<$@> containing
135			the exception. The default will simply C<warn> about the message and
136			continue.
137
138			=item c_cb => [$c_func, $c_arg]
139	root	1.1
140			Registers a C callback the be invoked whenever the async interrupt is
141			signalled.
142
143			The C callback must have the following prototype:
144
145	root	1.2	void c_func (pTHX_ void *c_arg, int value);
146	root	1.1
147	root	1.2	Both C<$c_func> and C<$c_arg> must be specified as integers/IVs, and
148			C<$value> is the C<value> passed to some earlier call to either C<$signal>
149			or the C<signal_func> function.
150	root	1.1
151			Note that, because the callback can be invoked at almost any time, you
152			have to be careful at saving and restoring global variables that Perl
153	root	1.4	might use (the exception is C<errno>, which is saved and restored by
154	root	1.2	Async::Interrupt). The callback itself runs as part of the perl context,
155			so you can call any perl functions and modify any perl data structures (in
156	root	1.4	which case the requirements set out for C<cb> apply as well).
157	root	1.1
158	root	1.6	=item signal => $signame_or_value
159
160			When this parameter is specified, then the Async::Interrupt will hook the
161			given signal, that is, it will effectively call C<< ->signal (0) >> each time
162			the given signal is caught by the process.
163
164			Only one async can hook a given signal, and the signal will be restored to
165			defaults when the Async::Interrupt object gets destroyed.
166
167	root	1.2	=item pipe => [$fileno_or_fh_for_reading, $fileno_or_fh_for_writing]
168	root	1.1
169	root	1.2	Specifies two file descriptors (or file handles) that should be signalled
170	root	1.1	whenever the async interrupt is signalled. This means a single octet will
171			be written to it, and before the callback is being invoked, it will be
172			read again. Due to races, it is unlikely but possible that multiple octets
173	root	1.2	are written. It is required that the file handles are both in nonblocking
174			mode.
175	root	1.1
176	root	1.6	You can get a portable pipe and set non-blocking mode portably by using
177			e.g. L<AnyEvent::Util> from the L<AnyEvent> distribution.
178
179			It is also possible to pass in a linux eventfd as both read and write
180			handle (which is faster than a pipe).
181	root	1.1
182	root	1.2	The object will keep a reference to the file handles.
183	root	1.1
184			This can be used to ensure that async notifications will interrupt event
185			frameworks as well.
186
187			=back
188
189			=cut
190
191			sub new {
192			my ($class, %arg) = @_;
193
194	root	1.6	bless \(_alloc $arg{cb}, @{$arg{c_cb}}[0,1], @{$arg{pipe}}[0,1], $arg{signal}), $class
195	root	1.1	}
196
197	root	1.2	=item ($signal_func, $signal_arg) = $async->signal_func
198	root	1.1
199			Returns the address of a function to call asynchronously. The function has
200			the following prototype and needs to be passed the specified C<$c_arg>,
201			which is a C<void *> cast to C<IV>:
202
203			void (signal_func) (void signal_arg, int value)
204
205			An example call would look like:
206
207			signal_func (signal_arg, 0);
208
209	root	1.2	The function is safe to call from within signal and thread contexts, at
210	root	1.1	any time. The specified C<value> is passed to both C and Perl callback.
211
212	root	1.2	C<$value> must be in the valid range for a C<sig_atomic_t> (0..127 is
213			portable).
214
215	root	1.1	If the function is called while the Async::Interrupt object is already
216			signaled but before the callbacks are being executed, then the stored
217	root	1.2	C<value> is either the old or the new one. Due to the asynchronous
218			nature of the code, the C<value> can even be passed to two consecutive
219			invocations of the callback.
220	root	1.1
221			=item $async->signal ($value=0)
222
223			This signals the given async object from Perl code. Semi-obviously, this
224			will instantly trigger the callback invocation.
225
226	root	1.2	C<$value> must be in the valid range for a C<sig_atomic_t> (0..127 is
227			portable).
228
229			=item $async->block
230
231	root	1.3	=item $async->unblock
232
233			Sometimes you need a "critical section" of code that will not be
234			interrupted by an Async::Interrupt. This can be implemented by calling C<<
235			$async->block >> before the critical section, and C<< $async->unblock >>
236			afterwards.
237
238	root	1.4	Note that there must be exactly one call of C<unblock> for every previous
239	root	1.3	call to C<block> (i.e. calls can nest).
240
241	root	1.4	Since ensuring this in the presence of exceptions and threads is
242	root	1.3	usually more difficult than you imagine, I recommend using C<<
243			$async->scoped_block >> instead.
244	root	1.2
245	root	1.3	=item $async->scope_block
246
247			This call C<< $async->block >> and installs a handler that is called when
248			the current scope is exited (via an exception, by canceling the Coro
249			thread, by calling last/goto etc.).
250
251			This is the recommended (and fastest) way to implement critical sections.
252	root	1.2
253	root	1.6	=item $async->pipe_enable
254
255			=item $async->pipe_disable
256
257			Enable/disable signalling the pipe when the interrupt occurs (default is
258			enabled). Writing to a pipe is relatively expensive, so it can be disabled
259			when you know you are not waiting for it (for example, with L<EV> you
260			could disable the pipe in a check watcher, and enable it in a prepare
261			watcher).
262
263			Note that when C<fd_disable> is in effect, no attempt to read from the
264			pipe will be done.
265
266	root	1.1	=cut
267
268			1;
269
270			=back
271
272	root	1.2	=head1 EXAMPLE
273
274	root	1.8	There really should be a complete C/XS example. Bug me about it. Better
275			yet, create one.
276	root	1.2
277			=head1 IMPLEMENTATION DETAILS AND LIMITATIONS
278
279	root	1.8	This module works by "hijacking" SIGKILL, which is guaranteed to always
280			exist, but also cannot be caught, so is always available.
281	root	1.2
282	root	1.8	Basically, this module fakes the occurance of a SIGKILL signal and
283			then intercepts the interpreter handling it. This makes normal signal
284			handling slower (probably unmeasurably, though), but has the advantage
285			of not requiring a special runops function, nor slowing down normal perl
286			execution a bit.
287
288			It assumes that C<sig_atomic_t> and C<int> are both async-safe to modify
289			(C<sig_atomic_> is used by this module, and perl itself uses C<int>, so we
290			can assume that this is quite portable, at least w.r.t. signals).
291	root	1.2
292	root	1.1	=head1 AUTHOR
293
294			Marc Lehmann <schmorp@schmorp.de>
295			http://home.schmorp.de/
296
297			=cut
298